An infrared shielding film can be applied to a wide variety of fields. For example, it is widely used as a film stacking to a window such as a heat ray reflecting film, in which heat ray reflection effect is imparted by attaching the film to facilities (base substrates), such as an outdoor window of a building or a window of a vehicle, to be exposed to the light of the sun for a long period of time. In addition, it is also used to increase mainly weatherability as a film for an agricultural vinyl greenhouse.
In a case where an infrared shielding film is used to be attached to a window of a building or a vehicle, it requires performance that effectively reflects near-infrared ray in order to function as the infrared shielding film, and also favorably permeates visible ray so as not to obstruct the view. However, in the related art, for example, since it strongly reflects only specific wavelength when visible light transmittance has a large irregularity, the film has coloring or color shading, and thus there is a problem that appearance is damaged.
As an infrared shielding film, various reflection films utilizing thin film interference are known, in which the reflection films have a configuration prepared by alternately laminating layers having mutually different refractive indexes. It is known that as the cause of coloring or color shading described above, the transmittance and reflectivity spectra of the reflection film shift into a short-wavelength side when a incident ray angle changes from normal incidence to rhombus incidence, and this fact is disclosed in the following Non-Patent Literature 1.
For this reason, when a near-infrared reflection film, in which there are no colorings regardless of the angle from which it is viewed, is provided, it was necessary to design a laminating configuration in advance so as to shift the reflection peak from a visible light wavelength region to a long-wavelength side in view of such a wavelength shift. However, such design narrows a reflection band, and thus it causes that the performance as an infrared shielding film cannot be improved.
In order to solve such a problem, the following Patent Literature 1 discloses a technique of overlaying a plurality of groups having different film thicknesses for a laminating structure prepared by alternately laminating the layers composed of two or more polymers. The narrow reflection peaks generated from the respective groups overlap by taking such laminating structure, and thereby heat ray reflection film that has a wide reflection band can be provided. However, there is a disadvantage of cost, in that for the method of alternately laminating the films by changing the film thickness in order, the number of films increase, and thus the number of the preparing processes increase and also the processes are complicated.
The following Patent Literature 2 provides an infrared filter, in which by having one periodic layer, that is, [(0.5M)H(0.5M)]n^ or [(0.5H)M(0.5H)]n^ (here, H represents a high refractive index material, M represents the layer having an optical film thickness of λ/4 of an intermediate refractive index material, and n^ represents the lamination being repeated n times), wavelet minute reflectivity variation (ripple) developed at a long-wavelength side and/or short-wavelength side of the reflection band is decreased, and thus uniform reflectivity is obtained in the whole wavelength region to be needed. However, for such infrared filter, the wavelength shift due to a change of viewing angle was not considered.